Zinc, iron, manganese and copper fertilizers are widely used in many parts of the world particularly in soils of high pH, such as the black soils. They are also popularly used in horticultural and cash crops to improve yields and quality of produce. Deficiencies of zinc, iron, manganese and copper in crops are usually corrected by application of the salts zinc sulphate, ferrous sulphate, manganous sulphate and copper sulphate respectively. Chelated forms of these micronutrients, e.g., iron-EDTA, and manganese-EDTA compounds are also in use as liquid sprays (J. J. Mortvedt, P. M. Giordano & W. L. Lindsay, 1972, Micronutrients in Agriculture, Soil Science Society of America, Madison, Wis.).
All such micronutrient fertilizers are water soluble salts. They are, therefore, easily leached out of the root zone or chemically transformed to compounds, which are not assimilable by plants. Dosages for micronutrient fertilizers are, therefore, enormous compared to the actual requirement. Thus, dosages for Fe as ferrous sulphate and Mn as manganous sulphate range from 20-40 kg/ha (J. J. Mortvedt, P. M. Giordano & W. L. Lindsay, 1972, Micronutrients in Agriculture, Soil Science Society of America, Madison, Wis.). The actual uptake of these nutrients is in the region of 0.2-1.0 kg/ha. Such overdoses are required because micronutrients are leached out and chemically transformed. Therefore, economics of micronutrient use is not always favourable. This limits widespread usage and restricts improvement in yields. The problem is even more acute in well-drained soils receiving high rainfall, where water soluble micronutrient fertilizers may show little response. Additional long-term problems are ground water pollution due to fertilizer leaching and toxicity hazards due to overdoses.
An efficient way to provide macro and micronutrients is by using slow-release fertilizers. Polyphosphate has been a natural choice for formulation of insoluble micronutrient fertilizers because of low raw material costs. Micronutrient polyphosphate fertilizers may be categorized as (i) water soluble polyphosphate liquid sprays (Van Wazer, J. R., 1966, Phosphorus and its compounds, Interscience, New York), (ii) long chain metaphosphates of ammonium or potassium with incorporated micronutrients (Volkovich, S. L., 1972, Journal of Applied Chemistry (USSR) Vol 45, p 2479), (iii) phosphate glass frits, glass (Roberts, G. J., 1973, American Ceramic Society Bulletin, Vol 52, p 383) and (iv) partially polymerized, water insoluble linear polyphosphates Ray, S. K., Varadachari, C. & Ghosh, K., 1993, Industrial & Engineering Chemistry Research, Vol. 32, p 1218; Ray, S. K., Varadachari, C. & Ghosh, K., 1997, Journal of Agricultural & Food Chemistry, vol. 45, p 1447).
Of these, only the last three categories are slow release forms. Frits are prepared by fusing sodium, potassium or ammonium dihydrogen phosphates together with micronutrient salts at temperatures between 800.degree. and 1400.degree. C. and then rapidly quenching the melt to produce a glass (Roberts, G. J., 1973, American Ceramic Society Bulletin, Vol 52, p 383; Roberts, G. J., 1975, American Ceramic Society Bulletin, Vol 54, p 1069; Austrian Patent No 326160 of 1975; U.S. Pat. No. 3,574,591 of 1971; U.S. Pat. No. 2,713,536 of 1974). The major disadvantage of the phosphate glass frits is that the availability of the nutrients is by slow hydrolysis of the glass and is highly dependent on the soil (pH, moisture content, temperature, etc.) and on the crop (rate of growth, physiological factors, variety, etc.). Consequently, where nutrient release by hydrolysis does not match plant uptake, the fertilizer is not effective. Moreover, the high temperatures involved in the synthesis of frits together with the corrosive conditions make these materials fairly expensive and unsuitable for general use.
Another type of phosphate based water insoluble fertilizer is the metaphosphate. Metaphosphates of calcium and potassium together with micronutrients have been proposed (Volfkovich, S. I., 1972, Journal of Applied Chemistry (USSR) Vol 45, p 2479). A Russian patent (SU 1270148 of 1986) describes the production of mixed metaphosphates based fertilizers produced at 500.degree.-880.degree. C. Drawbacks in the use of metaphosphates as fertilizers are similar to those with the frits. Metaphosphates may be more insoluble and hydrolyse even slower, producing compounds with very poor nutrient availability. Metaphosphates of the heavy metals are extremely insoluble and of little use as fertilizers.
The last category (iv) of slow-release fertilizer, which belongs to the bio-release type, is probably the most promising of those developed so far. Here, the micronutrient ions are in a chemical form wherein they are insoluble but are, however, completely bio-available (plant available). The processes for producing such phosphate based zinc and copper fertilizers are described in two Indian patents (Nos 172800 of 1990 and 177205 of 1991). The chemistry of zinc and copper phosphate polymerisation and the chemical nature of these fertilizers have also been described (Ray, S. K., Varadachari, C. & Ghosh, K., 1993, Industrial & Engineering Chemistry Research. Vol. 32, p 1218; Ray, S. K., Varadachari, C. & Ghosh, K., 1997, Journal of Agricultural & Food Chemistry, vol. 45, p 1447). Bio-release fertilizers of iron-manganese have also been described (DST PCT Application No. PCT/IN/04/00234 and Indian patent application No 971/DEL/03). Another patent (Indian Patent No. 194747 of 1999) describes the processes for production of slow-release cationic fertilizers.